Fiducial markers that can be located and recognized by an imaging system are useful in neurosurgery and other applications. For example, in one technique, multiple fiducial markers are screwed into the patient's skull to define recognizable landmarks that appear on a preoperative image of the patient's brain. Such a bone-anchored fiducial marker typically includes an externally threaded bone-screw portion, which is driven into the skull, and a threaded shaft that rises up and out of the skull from the bone-screw. The threaded shaft typically receives a screwed-on imagable sphere that is visible on a magnetic resonance imaging (MRI) image or computed tomography (CT) image. The multiple fiducial markers on the patient's skull define landmarks on preoperative images that are useful to the physician for planning entry coordinates and a trajectory to a target location in the brain. An image-guided workstation uses these preoperative images and planning to guide the neurosurgeon while actually performing the subsequent surgical procedure.
After the preoperative planning phase, the patient is brought into the operating room so that the planned surgical procedure can be performed. On the operating table, the patient's skull is clamped in a head-frame or otherwise immobilized. In order to use the preoperative images provided by the image-guided workstation to guide the surgeon during the procedure, the patient's skull must first be “registered” to the preoperative images. The registration creates an association between (1) the actual physical location of the fiducial markers on the patient's skull in the operating room and (2) the locations of the images of the fiducial markers visible on the preoperatively-obtained images.
According to one registration technique, a “wand” is used to perform the registration. The wand includes multiple light-emitting diode (LED) locators or reflective locators, which are visible to an infrared or other camera in the operating room. The camera is connected to the image-guided workstation. The locators define the position of the wand in the operating room, including the position of a sharp tip portion of the wand, which is in a known physical relationship to the locators. To register the patient, the imagable spheres are unscrewed from the fiducial marker shafts, and replaced by respective “divots” that are sized and shaped to receive the wand tip. These divots are screwed onto the fiducial marker shafts, such that the maximum depression point of the tip corresponds to the same location as the center of the imagable sphere when the imagable sphere was screwed onto the fiducial marker shaft. A reference divot is also present in the operating room at a known location, such as on the operating table or head-frame. During the patient registration process, the surgeon touches the wand tip to the reference divot, and then to each fiducial marker divot. This permits the image-guided workstation to correlate the actual physical location of the patient's skull to the preoperative images. The physician can then use the wand, in conjunction with the image-guided workstation, to locate an appropriate entry point and trajectory to the target in the brain.
One problem with the above registration procedure is the discomfort caused to the patient by the presence of the fiducial marker shaft extending upward from the bone-screw portion of the fiducial marker for receiving the screw-on imaging sphere and the screw-on divot. The upwardly-extending fiducial marker shaft can cause irritation to the patient's scalp. The presence of external threads on the shaft may increase the level of this irritation. Moreover, because there may be a long time period between preoperative imaging and the subsequent surgical procedure, the patient's scalp may be sewn up during the interim. Thus, the patient may experience such discomfort for an extended period of time. For these and other reasons, which will become apparent upon reading the following detailed description and viewing the drawings that form a part thereof, the present inventors have recognized an unmet need for fiducial marker devices, tools, and methods that reduce or avoid patient discomfort.